Coil form and method



l8, W STAHL COIL FORM AND METHOD Filed Oct. 30, 1963 I BY 077/6 v V lipI06 United States Patent 3,314,133 COIL FORM AND METHOD William F.Stain], 423 Abbottsford Road, Kenilworth, Ill. 60043 Filed Oct. 30,1963, Ser. No. 320,145 1 Claim. (Cl. 29155.57)

This invention relates to a coil form and method of fabrication, and,more particularly, to square or rectangular forms suitable for winding awire conductor thereon, the form possessing advantageous properties inuse as an inductor such as a transformer coil and other alternatingcurrent coils.

The invention is particularly useful, although not confined to so-calledmultiple wound or stick wound forms where several coils are woundsimultaneously on a relatively long length of coil form, say 724 inches,and then separated by sawing after the winding has been completed.

The principal requirements of such coil forms are dimensional stability,mechanical strength, inexpensiveness of manufacture, and the ability tobe sawed with a minimum burr.

In the past, i.e., for forty years or so, such coil forms have beenproduced of untreated kraft spirally wound tubing, which product hassatisfactorily met the low cost requirement. Untreated kraft paper ishygroscopic and subject to dimensional change on humidity changes orwetting. Hence, when it is spirally wrapped into a tube and exposed tothe Water in the adhesive, the paper changes dimensionally. The stressesthereby created cause a twist in the tube due to its spiralconstruction. Further, changing humidity conditions also causedimensional changes in the paper, further contributing to the abovedisadvantages as well as to day-to-day dimensional instabiliity.

Still further, since the untreated paper is soft and fibrous, when thecoils are sawed apart, a considerable burr is created. This makes itdifficult to insert the necessary steel laminations, this operationbeing termed stacking. The burr becomes more objectionable as thestacking operation becomes more mechanized.

It is, therefore, an object of this invention to provide a coil form anda method of fabrication thereof which overcomes these objections.

Another object of the invention is to provide a coil form made up ofunique laminae developing an advan- 'tageous stability and strength inuse.

Various expedients have been tried to overcome these poorcharacteristics:

(1) To minimize the hygroscopic qualities of the paper, the tubes havebeen treated with varnish. This helped the moisture absorption problemto some extent, but created a new dimensional problem due to theunavoida'ble build-up of beads of varnish on the inside tube surfaces.

(2) Fish paper, which is a tougher material than kraft, has been used asa liner on the inside surface to reduce the burr during sawing, sincefish paper saws with less burr than the more fibrous and softer kraft.However, this material is also hygroscopic and changes dimensionallyunder moisture at a quite different rate than the kraft, resulting thenin two materials with different shrinking rates opposing each other toaccentuate the twist problem.

More specifically, another object of the invention is to provide a coilform made up of layers of fibrous material such as kraft paper whereinat least one of the layers is saturated with a partially curedreinforcing plastic material such as a phenolic resin in the B stage.

A further object of the invention is to provide a-form 3,314,133Patented Apr. 18, 1967 resulting from spirally winding a dielectricpaper saturated with a B stage phenolic resin, and subsequently heatingthe form to cure the resin to the C stage.

Other objects and advantages of the invention may be seen in the detailsof construction and operation set down in this specification.

The invention is explained in conjunction with the accompanying drawing,in which:

FIG. 1 is a perspective view of the coil form of the invention shown inan intermediate stage of fabrication;

FIG. 2 is a sectional view taken along the line 2-2 of FIG. 1;

FIG. 3 is a view similar to FIG, 2 but showing the coil form at asubsequent stage of fabrication;

FIG. 4 is a view similar to FIG. 3 but of a modified form of theinvention;

FIG. 5 is a perspective view of a coil form in the process ofbeingwound; and

FIG. 6 is a perspective view of the wound coil form.

Referring now to FIG. 1, the numeral 10 designates generally a coil formor stick in the process of being formed. Normally, this is done on amandrel M (shown in dotted line) by spirally winding strips or ribbonsof paper as at 11 and 12. In the illustration given, the strip 11 is afirst continuous ribbon of kraft paper which may be saturated with aphenolic resin cured to the intermediate or B stage. The second ribbon12, which may be unimpregnated or saturated, as the case may be, iswound in overlapping, adhesive relation with the strip 11, both stripsbeing spirally wound. For example, the adjacent convolutions of theribbon 11 are designated 13 and 14, while the convolutions of the ribbon12 overlapping the convolutions 13 and 14 of the ribbon 11 aredesignated 15 and 16.

Equipment for performing this operation continuously is commerciallyavailable and usually includes some cutoff device whereby a stick of theorder of 24-28" is developed. Such a stick, generally designated A, isseen in FIG. 5 and is in the process of being gang wound by means ofconductors 17, 18, 19, 20, etc. The conductors are initially secured bymeans of an adhesive strip as at 21 and ultimately the stick A issevered along lines as at 17', 18', 19, etc., to develop the inductorgenerally designated I and seen in FIG. 6.

In FIG. 6, the coil form segment 10' provides a core for the winding,with the inner lead 22 having been picked from under the remainder ofthe winding. The outer lead 23 and the plurality of windings 24 aresecured by another adhesive strip 2 5.

From the foregoing, it will be appreciated that the abbreviated stickprovided in the coil form 10' is subjected to a number of differentstresses all tending to deform it from a predetermined configuration,i.e., circular, rectangular, square, etc. In the illustration given, thesquare form is utilized for simplicity of picturization.

According to the invention, the coil form developed by the oeprationpicture in FIG. 1 results in the discrete layers 11 and 12 which may beadhesively united by means of a thin layer of adhesive 26 (see FIG. 2).After the form 10 has been wound and severed to the desired length, itis transferred to a curing zone, either a continuous oven or abatch-style heater wherein the discrete layer .11 is subjected to heatto cure the phenolic impregnant thereof to the C stage. During thiscuring, the resin softens and a certain portion of the phenolicimpregnant migrates into the layer 12 as by coalescing or complexingwith the adhesive 26, eliminating the sharp line of distinction seen inFIG. 2this change in condition being reflected in FIG. 3 with the layersbeing designated 11' and 12'. I find that the resultant form, as seen inFIGS. 3 and 4,'

pregnated with phenolic resin, the intermediate layer 112 beinguntreated kraft. This makes a form having high resistance to shatter.Further, the phenolic impregnant surrounding the kraft keeps moistureabsorption down to a point that does not cause twist. Still further, theinside surface of the phenolic saws with a minmum burr.

It is believed that specific examples of the invention will further aidin the understanding thereof. For that purpose, the following are setdown.

Example I A stick 10 was wound in the fashion seen in FIG. 1 but withthree ribbons, the stick being generally rectangular and havingcross-sectional dimensions of l /s" by 1 /8" provided by three ribbonsof 2" wide, 0.010 thick dielectric kraft paper, using an aqueous basedpolyvinyl acetate adhesive, No. AC28 DN, obtained from Stein Hall ofChicago, which has about 53% solids, the adhesive being installedbetween the three layers of the core. The intermediate layer 112 wasconstructed of unimpregnated kraft paper as hereinbefore described,while the inner and outer layers 111 and 111a were made of phenolicimpregnated dielectric kraft paper. For this purpose, the resinimpregnant was Resinox 444 Varnish, obtained from Monsanto Chemical Co.The phenolic resin was partially cured, i.e., to the B stage. This stagecomprises a resin having a higher molecular weight than thatcharacteristic of the first stage, Bakelite A, made by heating phenoland formaldehyde in the presence of ammonia.

After the specimen had been prepared in the fashion of FIG. 1 and cut toa length of 24", it was subjected to a temperature of about 300 F. forabout one-half hour to to develop the higher molecular weight phenolicresin, i.e., the C stage. Lower curing temperatures for correspondinglylonger periods may be employed.

The specimen was found to be advantageously pliable, and upon beingwound in the fashion seen in FIG. 5, yielded slightly when the innerlead 22 was picked from the winding 24. Further, the specimen was ableto resist a pressure of 10 pounds.

Example II For this example, a two-layer specimen such as that seen inFIG. 3 was developed having cross-sectional dimensions of x 1", againwith 2" wide ribbons of 0.010 kraft. Only the layer 12 was impregnatedwith the phenolic resin of Example I. Here, the adhesive was No. 772, apolyvinyl acetate adhesive having a water base provided by theIndustrial Adhesive Company of Chicago, Illinois. This adhesive hasabout 50% solids. The stick 10 was cured, and again the cured productpossessed a desirable resistance to deformation upon being subjected tochanges in humidity. I find that the time of curing increases withheavier walls, thicknesses of 0.060 re- I quiring an hour or so, whilelower thicknesses, i.e., 0.030, requiring lesser time-one-half hour.Further, it manifested a desirable flexibility under the variousmanipulative operations performed upon it, while returning to originalshape thereafter.

both ribbons were saturated with the resin impregnant. The adhesiveemployed was the polyvinyl acetate aqueous based adhesive employed inExample I. This provided the maximum strength form-being 35% greaterthan the untreated kraft alone. In comparison, the form of Exple I has30% greater strength than straight kraft, while that of Example IIpossesses 2025% more strength. The cured form of Example III wascharcterized by minimum twist and minimum cutting burrs, after winding.The best application of this particular construction appears to be forrelatively large coils Where unusual rigidity is required. Thisparticular construction is slightly more shatterable in the lead pickingoperation than the forms of the other examples, particularly in the caseof smaller, thinnerwalled tubes.

Through the use of the invention, it is possible to realize significanteconomies by virtue of providing thinnerwalled tubing with superiorstrength, dimensional stability and freedom from objectionable burr whensawed, and, when desired, providing suitable flexibility for variousmanipulations. This is achieved through the use of preimpregnating thedielectric paper with a partially cured resin such as a B stage phenolicresin. The resin and the adhesive, during curing, intermix to form ahomogeneous dispersion whereby the advantageous enhancement of physicalproperties is achieved.

Of the adhesives, water-based adhesives having a relatively high solidscontent, of the order of about 50%, provide optimum results.

It will be appreciated that tubes can be made of allimpregnatedmaterial, or the impregnated material can be combined in varyingpercentages with untreated material to provide specially desiredresults. The impregnated paper, being relatively nonhygroscopic,provides a number of advantages. Strength is increased materially, whilewarping or twisting is substantially reduced. Even though the resultingtubes or coil forms are quite thin in wall thickness, they are notsensitive to variations in physical characteristics due to changinghumidity conditions. In fact, the dielectric character is increased andis fully maintained, in comparison to untreated kraft, for therecommended reduction in wall thicknesses. Still further, the more rigidphenolic paper cuts with less burr than other materials.

The specimen of Example II is about 30% phenolic resin in content and asindicated hereinbefore, has a 20 25% increase in strength over untreateddielectric kraft. The specimen of Example I is about 60% phenolic resinin content and possesses superior resistance to shatter. The Example Ispecimen appears to be the best general purpose construction in that itpossesses good strength, i.e., 30%, over plain kraft with the same wallthickness, and no lead picking problems are created even with thinwalls.

The use of the invention provides a saving in wire which, added to thespace saved by the thinner Walls, means that it is no longer necessaryto make a generous allowance over the lamination size to compensate foranticipated variations in construction. In terms of actual wire, thiscan amount to as much as 10%. The inventive construction also eliminatespressing or squeezing, since the dimensions can be maintained accordingto specification. Still further, the minimal cutting burr obtainedgreatly facilitates stacking and assists in lead picking-the absence ofburr permitting fine leads to be withdrawn without danger of rupture.

While in the foregoing specification a detailed description of anembodiment of the invention has been set down for the purpose ofexplanation, many variations in the details herein given may be made bythose skilled in the art without departing from the spirit and scope ofthe invention.

1 claim:

In a method of making a coil, the steps of spirally winding andadhesively uniting in the overlapped portions thereof at least tworibbons of paper in overlapping fash- F ion to develop a laminar tube,one of said ribbons being References Cited by the Examiner UNITED STATESPATENTS 6 Southerland 138-445 Henderson 138144 Scott 161 Mark et a1. 161Straka 9394 Fischer 154 LAVERNE D. GEIGER, Primary Examiner.

1,284,295 11/1918 Frederick 138-141 10 C-L-HOUCK,AssiSlantEmmiMr-

